The present invention relates to a radio receiver that uses channel estimation in receiving and demodulating a signal under a fading environment and, more particularly, to a radio receiver and noise estimated value correction method of correcting a noise estimated value.
In recent mobile communication, packet data traffic is becoming faster and increasing the capacity. Transportation is also speeding up simultaneously. Hence, when a radio device with a high transmission rate moves at a high speed, fading occurs, resulting in a large degradation in the characteristics.
To reduce the influence of fading, reference 1 (Japanese Patent Laid-Open No. 2004-7793) proposes a demodulation apparatus which obtains a channel estimated value by assigning weights to pilot signals contained in a received signal and averaging them and demodulates the received data by using the channel estimated value in mobile wireless communication such as CDMA (Code Division Multiple Access).
Reference 2 (Japanese Patent Laid-Open No. 2000-78104) proposes a CDMA synchronous detection channel estimation apparatus that accurately estimates a channel by adaptively controlling weighting coefficients in pilot signal averaging.
Reference 3 (Kawamura, Kishiyama, Higuchi, and Sawahashi, “Comparison Between Multipath Interference Canceller and Chip Equalizer with Other-Cell Interference Suppression in HSDPA in Multipath Channel”, Technical Report of IEICE, RCS2002-38, IEICE, pp. 123-128, 2002) and reference 4 (D. Falconer et al., “Frequency Domain Equalization for Single-Carrier Broadband Wireless System”, IEEE Communications Magazine, Vol. 40, No. 4, pp. 58-66, 2002) propose a method of suppressing characteristic degradation due to multipath fading by using a chip equalizer or frequency equalizer.
Reference 5 (“3G PP TS 25.214 V6.6.0”, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD) (Release 6), Chapter 5, pp. 13-30, 2005) proposes a method of controlling power by using the S/R ratio estimated value of a received signal.
A noise estimated value V calculated by using a channel estimated value is given by
                    V        =                              E            chip                    -                                    Ior              Ec                        ⁢                                          ∑                L                            ⁢                                                (                                      h                    1                                    )                                2                                                                        (        1        )            where Echip is the received signal power, h1 is the channel estimated value of the first path, L is the total number of paths, Ec/Ior is the ratio of pilot power Ec to total transmission power Ior.
When a mobile unit moves at a high speed, phase swinging occurs due to fading. If phase swinging occurs upon obtaining a channel estimated value by averaging a plurality of pilot symbols estimated from pilot signals, the received power is underestimated. Hence, the noise estimated value V calculated by using the channel estimated value becomes large, degrading the estimation accuracy of the signal-to-noise ratio (S/N) of the channel estimated value.
As a result, in a radio receiver that uses a noise estimated value and a channel estimated value to equalize a received signal, the equalization characteristic of the equalizer degrades. In a radio receiver that uses a noise estimated value and a channel estimated value for path search, the path assignment characteristic degrades. In a radio receiver that uses a noise estimated value and a channel estimated value to control power, the control characteristic degrades. For example, in a radio receiver including an MMSE equalizer, since a weight value obtained by using a noise estimated value and a channel estimated value is incorrect, the equalization characteristic degrades.
Reference 2 discloses a method of accurately estimating a channel, though no method of correcting a noise estimated value has been implemented yet.